System Stability

Introduction

The stability of a power system is its ability to remain in an operating equilibrium when subjected to the disturbances that are inevitable in any network made up of generating plant supplying loads. The disturbance may be small (e.g. caused by changes in load) or large (e.g. due to fault). After the disturbance a stable system returns to a condition of acceptable voltages and power flows throughout the system.

Figure 8.1 shows how power system stability may be divided into several aspects in order to make the problem easier to address. These include the loss of synchronism between synchronous generators (angle stability) either due to faults and large disturbances (transient stability) or oscillations caused by changes in load and lack of damping (dynamic or small-signal stability). Voltage instability can be caused by large induction motor loads drawing excessive amounts of reactive power during network faults when the network voltage is depressed or by a lack of reactive power when excessive real power flows in a network.

When the rotor of a synchronous generator advances beyond a certain critical angle, the magnetic coupling between the rotor and the stator fails. The rotor, no longer held in synchronism with the rotating field of the stator currents, rotates relative to the field and pole slipping occurs. Each time the poles traverse the angular region within which it could be stable, synchronizing forces attempt to pull the rotor into synchronism. It is usual practice to disconnect the generator from the system if it commences to slip poles, as pole slipping causes large current to flow and high transient torques.

Synchronous, or angle, stability may be divided into: dynamic and transient stability. Dynamic stability is the ability of synchronous generators, when operating under given load conditions, to retain synchronism (without excessive angular oscillations) when subject to small disturbances, such as the continual changes in load or generation and the switching out of lines. It is most likely to result from the changes

Electric Power Systems, Fifth Edition. B.M. Weedy, B.J. Cory, N. Jenkins, J.B. Ekanayake and G. Strbac. © 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.

Main forms of power system stability

Figure 8.1 Main forms of power system stability

in source-to-load impedance resulting from changes in the network configuration or system state and is a consequence of lack of damping in the power system. Often, this is referred to as small-signal stability.

Transient stability is concerned with sudden and large changes in the network condition, such as those brought about by short-circuit faults. The maximum power that can be transmitted, the stability limit, taking into account fault conditions is usually less than the maximum stable steady-state load.

The stability of an asynchronous motor load is controlled by the voltage across it; if this becomes lower than a critical value, induction motors may become unstable and stall. This is, in effect, a voltage instability problem. When the voltage at the terminals of an asynchronous (induction) motor drops, perhaps due to a fault on the power system, its ability to develop torque is reduced and the motor slows down. If the fault on the network persists the motor stalls and draws very large amounts of reactive power. These reactive power flows depress the voltage at the motor terminals further and the section of network has to be isolated. This form of voltage instability is a well known hazard in oil refineries and chemical plants that have large induction motor loads. Voltage instability can also occur in large national power systems when the loading of transmission lines exceeds the stable (approximately horizontal) section of the P-V curve, see Figure 5.21. Once the load of the transmission line approaches the 'nose' of the P-V curve instability can occur.

In a power system it is possible for either angle or voltage instability to occur, and in practice one form of instability can influence the other. Angle or synchronous stability has traditionally been considered more onerous and hence has been given more attention in the past. Recently, with the increasing use of static VAr compensators and the experience of large national black-outs caused by a deficit of reactive power, the study of voltage collapse has become important.

 
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